Directional ground relay



Jan. 5, 1937, R. D. EVANS 2,066,932

DIRECTIONAL GROUND RELAY Filed Dec. 28, 1955 2 Sheets-Sheet 1 WITNESSESQ Y INVENTOR Roberzfl E0005 Patented Jan. 5, 1937 UNITED STATES PATENT OFFICE DIRECTIONAL GROUND RELAY sylvania Application December 28, 1933, Serial No. 704,310

8 Claims.

My invention relates to protective apparatus for electric circuits and particularly to directional ground relays for polyphase alternating current transmission and distribution circuits.

Although not limited thereto, my invention is particularly applicable to ground relays of the type consisting of independent power-directional and current responsive elements as usually applied for clearing ground faults in polyphase circuits having at least one neutral point grounded solidly or through a reactor or resistor. In such relays as commonly constructed, the power directional element is energized in accordance with a vector product of zero sequence voltage and zero sequence current, or in accordance with a residual power product of the protected circuit.

In order to obtain the zero sequence voltage necessary for the power directional element of such relays, either of two connections may be used dependent upon the type of power apparatus available at the point at which the relay is applied. If a polyphase transformer or bank of transformers having star connected high voltage windings with neutral grounded and delta connected low voltage windings is available, the equivalent of zero sequence voltage may be obtained by a single current transformer connected in the neutral ground conductor of the high voltage windings. If no such star-delta apparatus is available, three insulating potential transformers must be used to obtain the zero sequence voltage.

As the cost of an insulating potential trans- D former for use at transmission voltages is of the order of to times the cost of a directional ground relay and approximately double the cost of an insulating current transformer of the same voltage rating, it will be apparent that the overall cost of a ground relay installation may be considerably reduced by eliminating one or more of the three insulating potential transformers ordinarily used.

It is accordingly an object of my invention to provide a novel directional ground relay for three-phase circuits which shall require a maximum of two insulating potential transformers.

Another object of my invention is to provide a novel directional ground relay having a poten- 0 tial winding which may be energized from the low voltage side of any polyphase transforming apparatus available at the relaying point.

Another object of my invention is to provide a novel directional ground relay which shall respond to unsyrmnetrical line-to-line faults as well as ground faults.

A further object of my invention is to provide a novel directional ground relay having a directional element energized in accordance with variables which may pass delta connected transformer windings.

Other objects of my invention will become evident from the following detailed description taken in conjunction with the accompanying drawings, in which Figure 1 is a diagrammatic view of a relay embodying my invention as applied to a highvoltage power system, and

Fig. 2 is a diagrammatic view of a modification of the relay shown in Fig. 1.

Referring to Fig. 1 in detail, a generator I is connected by means of any suitable transforming apparatus, shown as a delta-delta connected bank of transformers 2 to a high voltage bus 3. A pair of transmission circuits or high voltage feeders 4 and 5, for supplying power to any suitable receiving apparatus 6 and 1 respectively, indicated as grounded-neutral star connected transformer windings, are connected to the high voltage bus 3 by means of a pair of circuit breakers 8 and 9, respectively.

The circuit breaker 8 may be of any suitable high voltage type having a trip coil 8a and condenser type or oil filled type bushings 81) for insulating the high voltage circuit breaker parts, in a manner well understood in the art. One set of the bushings 8b is provided with a set of bushing type current transformers, shown diagrammatically at I0, the purpose of which will be hereinafter explained. The circuit breaker 9 may be of the same general construction as the circuit breaker 8, but for simplicity is shown in more diagrammatic form.

The trip coil 8a is connected in a trip circuit with any suitable source of tripping current, indicated as a battery I I and. the contact members I3a of a directional ground relay I2, constructed in accordance with my invention. The ground relay I2 comprises an overcurrent element I3 and a power directional element I4, which may be mounted in the same case in the usual manner.

The overcurrent element I3 is preferably of the induction disc type having a lower pole winding I31) and a pair of upper pole windings I30 mounted upon angularly displaced pole members in the usual manner. The upper pole windings I30 are normally deenergized but may be connected to an auxiliary winding I3d to be inthe overcurrent element 13 closes to trip the circuit breaker 8.

If an unsymmetrical fault occurs on the high voltage circuit 5, the direction of negative sequence power fiow at the relaying point is opposite to that described above and the direction element l4 remains open.

It will be apparent that negative sequence power flow and zero sequence current occur under the following fault conditions: single phaseto-ground faults, double phase-to-ground faults at the same location, and three-phase-to-ground faults where the fault impedances are unsymmetrical. For phase-to-phase faults, there may be considerable flow of negative phase sequence power, but no zero phase-sequence current flow. For three-phase symmetrical faults, both the negative sequence and zero sequence power fiow are zero. It will therefore be seen that the relay l2 operates under substantially the same fault conditions as ground relays of the usual zero sequence directional type.

In the system described above, the voltage of the generator I may be of medium value such as 6600 or 13,200 volts, whereas the voltage of the bus 3 and circuits 4 and 5 may be a transmission voltage such as 66 KV. The voltage filter I5 is connected on the low voltage side of the transformer bank 2 in order to avoid the use of high voltage insulating potential transformers. As negative sequence voltage is transmitted by delta, star, open V or Scott connected transformer windings, the connection shown in Fig. 1 may be used wherepower transformer banks connected in any usual manner are available. Insulating current transformers l6a and lBb are used in preference to bushing type current transformers to supply the filter l5, as the volt-ampere capacity of the former type is higher than that of the latter, and the burden imposed by the filter impedances is comparatively large. It will be obvious however, that the circuit shown may be modified in many ways.

Referring to Fig. 2, which shows a modification of my invention employing a current phase sequence filter of the type disclosed in the copending application of W. A. Lewis mentioned above, a relay 12 having an overcurrent element l 3 and a power directional element [4, constructed in the same manner as the relay 12 of Fig. 1, is arranged to control a high voltage circuit 4 by means of a circuit breaker 8 and battery II as in Fig. 1.

The power directional element [4 is energized by means of a negative sequence voltage filter l5 which may be of the same type as the filter 15 of Fig. 1, and a negative sequence current filter H.

The filter l1 comprises a bank of insulating current transformers Ila having their secondary windings connected in star to a pair of 5:5 ratio low voltage current transformers l 1b, a resistor "c and a reactor Ild. The primary windings of the current transformers [1b are connected in series with the secondary winding of one of the current transformers Ila, and the secondary windings of the current transformers llb are connected in reversed open V across the star circuits, beyond the primary windings of the current transformers Ill). The resistor [1c and the reactor We; are connected in parallel to the secondary windings of the current transformers lib. The impedance of the reactor l'ld is equal to the impedance of resistor Il'c rotated through 60 in the lagging direction, and for maximum efficiency'is equal to .866 times the impedance of the current windings Me of the power directional element 14, as in Fig. 1. A neutral return conductor He is connected in the manner shown to include the lower pole winding I3b of the overcurrent element I3.

The normal phase relationship of star currents and voltages of the circuit 4 for power flow in the normal direction is indicated by reference characters a, b, and c. As explained in the copending application of W. A. Lewis mentioned above, a filter constructed in the manner described above produces zero sequence current in the neutral return conductor He and negative sequence current in the current windings I40. The phase position of the negative phase sequence current output of the filter I1 is the same as that of the current filter I6 of Fig. l.

The operation of the apparatus shown in Fig. 2 will be obvious from that described above in connection with Fig. 1.

I do not intend that the present invention shall be restricted to the specific structural details, arrangement of parts or circuit connections herein set forth as various modifications thereof may be effected without departing from the spirit and scope of my invention. I desire therefore that only such limitations shall be imposed as are indicated in the appended claims.

I claim as my invention:

1. In an alternating current system of transmission and distribution having a first polyphase power circuit and a second polyphase power circuit connected by power transformer apparatus, said transformer apparatus having polyphase windings connected to pass positive sequence power components and prevent the passage of zero sequence power components, ground fault protective means for said first circuit including a directional element having potential coil means energized in accordance with potential of sequence other than zero derived from said second circuit and current coil means energized in accordance with current of said sequence other than zero derived from one of said power circuits.

2. In an alternating current system of transmission and distribution having a first polyphase power circuit and a second polyphase power circuit connected by power transformer apparatus, said transformer apparatus having polyphase windings connected to pass positive sequence power components and power components of a different sequence, ground fault protective means for said first circuit including a directional element having potential coil means energized in accordance with potential of said different sequence derived from said second circuit and current coil means energized in accordance with current of said different sequence derived from one of said circuits, and an overcurrent element energized in accordance with an unbalanced current condition derived from said first circuit.

8. In a directional ground relay for polyphase alternating current circuits, a directional element responsive to a vector product of negative phase-sequence current and negative phase-sequence voltage, and an overcurrent element responsive to zero phase-sequence current, said elements being connected to effect a control operation only when said vector product is in a predetermined directional condition and a zero phase-sequence component of current exceeds a predetermined value. 7

4. In a directional ground relay for polyphase alternating current circuits, a directional element of the watt responsive type having a potential winding and a current winding, means for energizing said potential winding in accordance with a negative symmetrical voltage component, means for energizing said current winding in accordance with a negative symmetrical current component, an overcurrent element, and means for energizing said overcurrent element in accordance with ground current, said elements being connected to effect a control operation only when said watt element is energized in a predetermined direction and a ground current condition exceeds a predetermined value.

5. In protective apparatus for a polyphase alternating current circuit, a circuit breaker, a relay element responsive to a vector product of negative phase-sequence current and negative phase-sequence voltage of said circuit, said element having contact members operable in response to a predetermined directional value of said product, an overcurrent element having contact members operable in response to a predetermined current condition, means controlled by said first-mentioned contact members for energizing said overcurrent element in accordance with the zero phase sequence current of said circuit, and means including said contact members of said overcurrent element for operating said circuit breaker.

6. In an alternating current system of transmission and distribution having a three-phase high voltage circuit, directional ground protective apparatus for said circuit comprising a circuit breaker, a power directional relay element having a potential winding and a current winding, a pair of insulating potential transformers connected to said circuit, energizing means for said potential winding including a negative phase-sequence voltage filter energized from said potential transformers, means for energizing said current winding in accordance with the negative phase-sequence current of said circuit, an overcurrent relay element responsive to the zero phase-sequence current of said circuit, and means including said relay elements for operating said circuit breaker in response to simultaneous directional energization of said directional element and energization of said overcurrent element above a predetermined degree.

7. In an alternating current system of transmission and distribution having a three-phase high voltage circuit and having polyphase power transforming means including high vol age windings connected to said circuit and low voltage windings, directional ground protective apparatus for said circuit comprising a circuit breaker, a directional relay element having a potential winding and a current Winding, energizing means for said potential winding including a negative phase-sequence voltage filter energized from said low voltage windings, means for energizing said current winding in accordance with the negative phase-sequence current of said circuit, an overcurrent relay element, means for energizing said overcurrent relay element in accordance with the zero phase sequence current of said circuit, and means including said relay elements for operating said circuit breaker in response to simultaneous directional operation of said directional element and energization of said overcurrent element above a predetermined degree whereby said high voltage circuit is protected against unsymmetrical ground faults in a predetermined direction without the use of high voltage insulating potential transformers.

8. In protective apparatus for a polyphase alternating-current circuit, a circuit breaker, a directional relay element having contact members, an overcurrent relay element having contact means, energizing means for said relay elements including means for energizing said directional relay element in accordance with a vector product of negative sequence current and negative sequence voltage of said circuit, said energizing means including means controlled by said contact members for energizing said overcurrent element in accordance with a current condition derived from said circuit, and means controlled by said contact means for causing said circuit breaker to open when said vector product is ina predetermined directional condition and said current condition exceeds a predetermined value.

ROBERT D. EVANS. 

